Internal combustion engines may include water injection systems that inject water into a plurality of locations, such as into an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Engine water injection provides various benefits such as an increase in fuel economy and engine performance, as well as a decrease in engine emissions. In particular, when water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to evaporate the water, leading to charge cooling and engine dilution. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, improved wide-open throttle performance, decreased heat transfer losses, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx emissions, while a more efficient fuel mixture (less enrichment) may reduce carbon monoxide and hydrocarbon emissions.
Engine control systems may select when to use water injection based on engine operating conditions, such as engine knock limitations. One example approach is shown by Surnilla et al. in US 20130218438. Therein, water usage for dilution control relative to knock control is adjusted based on combustion stability limits. Another example approach is shown by Leone et al. in US 20140202434. Therein water injection is used when the engine load is higher than a threshold or the engine is knock limited.
The inventors herein have recognized that the maximum fuel economy benefits of water usage may be limited by the availability of water on-board the vehicle. In particular, the water supply may be limited based on how much water can be generated on-board the vehicle via-a-vis how much water is required for knock control, dilution control, catalyst temperature control, etc. As an example, if water injection is enabled for catalyst temperature control, due to the high water consumption rate during catalyst temperature control, there may be insufficient water available for knock control. As a result, spark may need to be retarded for knock control. The fuel penalty associated with the use of spark retard may offset or even outweigh the fuel economy benefit associated with water usage for catalyst temperature control.
In one example, the above issues may be addressed by a method for an engine comprising: comparing a current water level in a water reservoir and a predicted water level in the reservoir over a vehicle drive to a plurality of threshold water levels; and injecting water from the reservoir into the engine responsive to each of engine knock, dilution demand, and exhaust temperature based on the comparison. In this way, water usage may be prioritized if water availability is limited.
As an example, an engine may be configured with a water injection system that enables water to be injected into one or more engine locations, such as into an intake manifold, into an intake port, or directly into an engine cylinder. The water injection system may include one or more water injectors coupled to the different locations, as well as a water reservoir supplying water to the injector(s). The water reservoir may be manually refilled by a vehicle operator. Additionally, the water reservoir may be coupled to a water collection system that opportunistically refills the reservoir with water generated on-board the vehicle. For example, water in the form of condensate may be retrieved from one or more vehicle components, such as an EGR cooler, an AC evaporator, an exhaust heat exchanger, a charge air cooler, a vehicle external surface, etc. An engine controller may assess engine operating conditions and determine respective amounts (and locations) of water to inject into the engine for each of knock control, exhaust temperature control, as well as to meet engine dilution demand. The controller may also retrieve a current water level in the water reservoir and predict an expected water level in the reservoir over a vehicle drive cycle based on current and predicted rates of water generation as well as current and predicted rates of water usage (e.g., for knock, dilution, and exhaust temperature control). Based on the current water level and the predicted water levels over the drive cycle, the controller may assign a priority value to each of the respective uses of water, and determine the amounts to be injected for knock control, exhaust temperature control, as well as to meet engine dilution demand. In addition, based on the current water level and the predicted water level (as well as the trend for water availability from the current water level to the predicted water level), the controller may determine a plurality of water level thresholds, and compare the amounts to be injected to those water level thresholds. The selected amount for the current operating conditions may then be injected based on the comparing. As one example, when the water level is already low and/or is predicted to fall over the drive cycle, the water injection amount for knock control may be given highest priority and sufficient water may be injected to ensure good knock control, but no water may be injected for exhaust temperature control or for dilution demand. The prioritization is used in order to achieve the highest engine efficiency benefit per unit of water, when the water supply is limited. If water injection is not used for meeting the dilution demand, then the opening of an EGR valve may be adjusted based on the selected water injection amount to meet the dilution demand. Likewise, if the water level is currently very low and not predicted to rise, and water injection is not used for knock control, then spark timing may be retarded based on the selected water injection amount for knock control. In still further examples, the water level threshold required for enabling water injection for dilution control may be lowered when the predicted water level increases from the current water level, and raised when the predicted water level decreases from the current water level. As a result, water usage for dilution control can be limited when water availability is expected to drop.
In this way, the fuel economy benefit of water injection can be maximized, particularly when operating with a limited water supply. By assigning a priority value to water injection amounts applied for distinct engine operating conditions, and injecting the water amount based on the highest priority value when the water supply is limited, the efficiency benefit per unit of water injected can be substantially increased. By stopping or reducing the injection of water during operating conditions having a lower efficiency benefit, engine performance can be maintained elevated until the water reservoir becomes empty. By also varying the selection of the water injection amount based on the estimated quality of water refilled into the water reservoir, water usage benefits can be extended over a wider range of engine operating conditions, even when the water supply is of poor quality.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.